THE EFFECT OF CO₂ SUPPLY ON THE YIELDING AND ECONOMIC CONSEQUENCES OF THIS TREATMENT IN GLASSHOUSE CULTIVATION OF TOMATOES

Eugeniusz Kołota, Zbigniew Ślązak

ABSTRACT

The studies conducted in a glasshouse of the Establishment of Horticultural Production (PPO) at Siechnice were aimed to determine the reaction of tomatoes grown on mineral wool to CO₂ enrichment of the atmosphere and to calculate the profitability of applying this treatment in wholesale production. The studies used liquid gas accumulated in pressure containers. After being evaporated, the gas was carried in liquid form to plants by means of a system of PE pipes. The studies adopted three levels of CO₂ content in the glasshouse atmosphere, namely 0.06-0.08% with ventilators closed, 0.04-0.05% with ventilators slightly opened, and 0.03-0.04% with ventilators fully opened.

Key words: tomato, glasshouse cultivation, CO₂ enrichment, yield, profitability of cultivation.

INTRODUCTION

In natural conditions for the majority of plants it is not CO₂ concentration in the atmosphere that constitutes a factor limiting the intensity of photosynthesis. This fact is taken advantage of in glasshouse cultivation, where plants are supplied with carbon dioxide. Within a certain range, doubling CO₂ content in the air also doubles the intensity of photosynthesis, which is of significant importance for the production of biomass [13].

The first Polish experiments on supplying tomatoes with CO₂ conducted by Skierkowski [9] showed that a 0.1-0.15% increase of its concentration in a glasshouse secured an increased yield of fruit by 30% and of the early yield by 80%. A similar range of CO₂ concentrations in the cultivation of this plant is also recommended by Libik et al. [7], Sowiński [11]. Libik and Repelewicz [6]. According to Mortensen [8], the optimum CO₂ concentration for the majority of plants is slightly lower and is placed within the range between 0.06-0.09%. On the other hand, Cydendambajew [2] points out that the optimum CO₂ concentration is related to the stage of tomato growth and is equal to 0.06-0.08% in the phase of seedlings production, and 0.1-0.15%, when the plants are fully developed. CO₂ enrichment of plants can also be profitable in summer with open ventilators, the aim of which is to keep its concentration at the level up to 400 ppm [10]. This refe rs especially to no soil cultivation, where in the afternoon the level of CO₂ can decrease below 100 ppm, which is the value below the compensation point.

Apart from an increase of the fruit yield, especially the early yield, another effect of CO₂ supply is an increase of sugar content in the fruit and a considerable drop of acids concentration and – resulting from it – improvement of organoleptic valours [6]. At the same time attention is drawn to the fact that profitability of this treatment is conditioned by other factors, including the light, air humidity, fertilization and temperature, which should be by 2-3°C higher that in cultivation of plants that are not enriched [3, 6].

Despite encouraging results of studies on CO₂ enrichment this treatment did not arouse much interest in horticultural practice. It began to be used only in the 1980’s in West European countries, and lately also in Poland, especially in cultivation on inorganic subsoils. A possibility of using CO₂ free from impurities in the compressed form in cylinders, improvement in the equipment for dosing and controlling the concentration as well as greater air-tightness of glasshouse construction contribute to the popularity of enriching plants with CO₂.

The purpose of the studies was to determine the effect of CO₂ application on the production and economic effects of glasshouse wholesale production of tomatoes.

MATERIALS AND METHODS

The studies were conducted at the Establishment of Horticultural Production Siechnice near Wrocław, in the conditions of wholesale production of glasshouse tomatoes, where in 1991 this vegetable started to be cultivated on mineral wool with computer control of the climate, fertilization and irrigation. Beginning with 1997, plants were enriched with carbon dioxide, which was supplied to the glasshouse atmosphere.

PPO Siechnice uses a prolonged method of tomato cultivation, where seed sowing is performed on December 10-15, while the seedlings cultivated in blocks are planted from mats to mineral wool 40 days after the sowing date and plant density is 2.4 plants per 1 m². Plants are led in one stem and the topping takes place at the beginning of September. Large scale introduction of biological methods of protecting plants from greenhouse while fly and red spiders made it possible to use bumble-bees for plant pollination. The fruit are picked 3 times a week. The final harvest was joined with closing down the plantation is in the middle of December.

Plant enrichment is conducted throughout the period of tomato cultivation on a permanent place, with the use of liquid CO₂. The whole system of enriching the greenhouse atmosphere in this gas is comprised of the following:

• pressure containers POLKOMAT 16 or 25, where liquid CO₂ is stored, and a system of ventilator evaporating dishes with the efficiency of 1500 m³/h for vaporizing this gas;

• a system distributing gas CO₂ to particular greenhouses by means of PCV pipes with 50 mm;

• a system distributing and dosing CO₂ in particular greenhouses. The distributing wires are made of PCV pipes with 40 and 32 mm, while the dosing elements are made of PE pipes with 20 mm, with an opening of 1.5 mm every 8 m., placed along the plant rows. The working pressure in the distributing pipes is 1.6-2.0 bars, and in the dosing wires 0.5 bar;

• a steering system, the function of which is to obtain a definite level of CO₂ in the greenhouse. Its most important element is an analyzer of gas concentration of PRIVA Company. It cooperates with PRIVA CD 750 computer steering the whole process of tomato production.

Three levels of CO₂ content were adopted in the greenhouse atmosphere in the production of tomato:

• 0.06-0.08% with ventilators closed completely,

• 0.04-0.05% with ventilators opened up to 5%,

• 0.03-0.04% with ventilators opened > 10%.

CO₂ dosing was used in the spring period between 9.00 am.-3.00 pm, and in the summer and autumn periods between 7.00 am-5.00 pm.

The total area of tomato cultivation under the studies was 4 ha every year. The results obtained in 1994-1996 helped to estimate the yielding of plants that were not enriched, while the results from 1998-2000 were achieved with CO₂ enrichment. The year 1997 was not considered in the studies because the cultivation was damaged by flood.

The mean weekly yield of fruit from the whole period of tomato yielding was used to evaluate the commercial yield. An analysis of price tendencies on the greenhouse vegetables market was the basis to distinguish the very early and early yields. On the basis of the data from all the examined years, the very early yield, calculated since the first permanent reduction of prices for fruit, was considered to mean all the crops gathered in the period up to the 19^th week of the calendar year. On the other hand, the early yield was considered to mean the crop gathered since the above time until the next reduction of prices in the third ten days’ period of June (a period of 19^th-25^th weeks of the year).

In the economic estimation – on the basis of weekly crops of fruit signifying the means for the years with and without CO₂ enrichment and the market prices of the year 2000 – the gross income from tomato cultivation was calculated. Besides, the costs of enrichment were worked out and they consisted of the price of CO₂ purchase, the cost of leasing the tanks and evaporating dished, amortization, expenditure of electrical energy, the costs of services, repairs and maintenance. These data were used to calculate the economic indexes, namely the index of profitability and the relative costs of CO₂ enrichment in relation to additional incomes achieved after the treatment was introduced.

From among the weather data the paper chose to present only the measurements of insolation as a factor that is not controlled in greenhouse cultivation (table 1). Calculating the mean monthly insolation in the years 1994-1996, when the plants were not enriched, and in the years when they were (1998-2000), it can be stated that it was slightly higher for the tomatoes that were not enriched. This means that when the other elements of greenhouse micro-climate was kept on the same level year by year, an increased yield of fruit in the years with CO₂ enrichment was solely the effect of this treatment.

RESULTS

In all the analyzed years the fruit harvest began in the 13^th week of the calendar year, i.e. on the first days of April. The year 1999 was an exception because then the beginning of the harvest was a week earlier. In all the examined years very intensive ripening of the fruit and consequently increased crops were observed beginning with the 16^th week of the year (third 10-day period of April). The yielding was decreased after 38^th-39^th week in the variant without CO₂ enrichment, and after 44^th-45^th week in the variant with CO₂. The harvest was finished after the 48^th week that is in the first half of December, independent of the use of CO₂ application.

The results concerning tomato yielding showed slight differentiation in the successive vegetation seasons. That is why they are presented as mean figures for the period of 3 years. It follows from the data in figure 1 that supplying plants with carbon dioxide gave a significantly higher yield, mainly at the beginning of the harvest, between the 13^th and 19^th weeks of the calendar year. In the successive period the effect of CO₂ enrichment was differentiated; the plants supplied with CO₂ showed both increases and decreases of the yield in relation to the cultivation without CO₂ enrichment. In the autumn period, however, the studies again observed high effectiveness of this treatment in the 39^th-46^th weeks.

In sum, percentage increase of the yield calculated from the beginning of the harvest in the successive weeks of the year (figure 2) showed very high efficiency of this treatment during the first 3 weeks of yielding, when it reached 250% in relation to the cultivation without CO₂ enrichment. In the successive 4 weeks the yield was lowered to 30% showing further slow decrease in the summer period and increase in autumn.

The data contained in table 2 and referring to the whole period of cultivation show that in conversion per 1 m², supplying plants with CO₂ caused an increase of the commercial yield of fruit from 38.1 to 45.5 kg, of the early yield from 8.8 to 9.7 kg, and of the very early yield from 5.0 to 7.5 kg. A high, up to 50% increase of the very early yield points at especially high effectiveness of the treatment at the beginning of the cultivation and fruit harvest of tomato. That increase was also significant in reference to the overall commercial yield and it was 19.4%.

It can be stated on the basis of the data from table 3 calculated from the data of the year 2000 that the purchase of CO₂, including the costs of leasing evaporating dishes and tanks, constituted the highest proportion in the costs of plant enrichment. The following important item in the costs was servicing the enriching devices. Its proportion in the total costs was 35.5%. The financial outlays including amortization, repairs and maintenance of the dosing and controlling devices constituted a relatively small percent. The total cost of enrichment per 1 ha of tomato cultivation was determined to be 51 037.20 PLZ, which – with simultaneous increase of the gross income by 306 123.50 PLZ (22.3%) – ensured the clear income of 255 086.30 PLZ from 1 ha in consequence of applying CO₂ enrichment (table 4). High profitability of enriching greenhouse tomatoes with carbon dioxide is proved by the index of relative co sts, which is 16.67% and the index of profitability, which reaches 499.80%.

DISCUSSION

The new technology of production, using the mineral wool as the subsoil, the system of plant fertigation and steering the greenhouse climate of PPO Siechnice made it possible to achieve an increase of the yield of tomato fruit in the prolonged cultivation up to 38.1 kg/m², which made the cultivation profitable. Applying CO₂ in plant enrichment, which began in 1995 and is now used in all greenhouse cultivation, was considered to be a condition of further improvement of the productive and economic effectiveness.

The studies conducted in the years 1994-2000 showed that the increase of tomato yield achieved under the effect of supplying plants with carbon dioxide in the conditions of wholesale production was lower in relation to the increase obtained on an experimental scale by Skierkowski [9], Hartz, et al. [4], Libik and Repelewicz [6] and quoted by Sowiński [11].

This can be accounted for both by greater difficulties in achieving the optimum conditions in a huge wholesale object and by the applied CO₂ concentration, which was kept at the level of 0.06-0.08% with closed ventilators, and only 0.03-0.04% with open ventilators, doing away with the deficit of the gas intensively utilized in the process of photosynthesis.

An increase of the commercial yield of fruit from 38.1 to 45.5 kg/m² should be considered to be fully satisfying, especially when one considers the fact that the highest increase was observed at the beginning of fructification. It turned out that the increase of the very early yield achieved during the first 7 weeks of yielding (until the middle of May) reached 50%, which – considering very high retail prices – had a significant influence on the application of this treatment. When Libik and Repelewicz [5] applied CO₂ enrichment in the concentration 800 ppm, they found out an increase of the yield of early fruit within the range of 77-98% depending on the year, which has a decisive effect on the financial outcome if one takes into account a 27% increase of the total commercial yield and more favourable price relations at the beginning of the crop. High efficiency of the treatment of CO₂ enrichment at that time can be accounted for by the fact that young plant s show a more positive reaction towards the content of this gas in the atmosphere, because in older plants a genetically conditioned drop of photosynthesis intensity takes place [1, 12].

At PPO Siechnice enrichment with CO₂ contributed to higher gross income by 22.3%, which – with relatively low costs of the application of the treatment – gave a very high profitability rate of 499.8%. Hartz et al. [4] also point at high profitability of CO₂ enrichment. They establish its costs at 10% of the total costs of tomato production. On the other hand, Skierkowski [9] found out a 20% increase of the value of the fruit yield in the spring cultivation and no effectiveness of the treatment in the autumn cultivation. It should be emphasized, however, that these results were obtained on traditional subsoil, where no radical drop of the level of CO₂ caused by intensive photosynthesis process in the greenhouse atmosphere takes place.

CONCLUSIONS

Results of the studies conducted in the years 1994-2000 and concerning profitability of enriching greenhouse tomatoes with carbon dioxide point at the following conclusions:

1. CO₂ enrichment on a permanent place on the subsoil of mineral wool gave increased commercial yield of fruit from 38.1 to 45.5 kg/m² (by 19.4%)

2. A positive effect of enriching the atmosphere with CO₂ was especially revealed in the case of the very early yield of fruit, which had a decisive importance for the economic profitability. That yield gathered from the beginning of April till the middle of May was for the period of 3 years higher by 50% in relation to the cultivation without enrichment.

3. The influence of CO₂ enrichment on the economic effect of tomato cultivation was shown in increased clear income by 22.3%

4. The rate of relative value of additional costs of CO₂ enrichment was 16.67%, while the rate of profitablity reached the value of 499.8%.

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